KINETICS AND OPTIMIZATION OF PHENOL UPTAKE FROM AQUEOUS SOLUTION USING AGRICULTURAL WASTES
Department: Chemical Engineering
Iheanacho, C. O.
Nwabanne, J. T.
The uptake of phenol from simulated aqueous solution using agricultural wastes as adsorbents is the focus of this research work. The agricultural wastes used were corn cob and rice husk which were modified with tetraoxophosphate V acid (H3PO4) and carbonized to give corn cob activated carbon (CCAC) and rice husk activated carbon (RHAC) respectively. The surface area of the adsorbents was determined using the Brunauer-Emmett-Teller (BET) nitrogen absorption method. Other physical properties of the adsorbents such as fixed carbon, bulk density, moisture content, volatile matter etc were determined using the method of Association of Analytical Chemistry (AOAC). Instrumental characterization was carried out using the Scanning electron microscopy (SEM) to examine the surface morphology and the Fourier Transform Infra-Red (FTIR) spectrophotometer to determine the functional groups present in the adsorbents. The effects of batch adsorption operational parameters such as contact time, initial phenol concentration, temperature, adsorbent dosage and pH on the phenol uptake were investigated. The adsorption equilibrium was evaluated by fitting the experimental data to nine linear isotherms and tennon linear isotherms. Nine linear and non linear kinetic models were employedin the kinetic study. Eight error terms were used to determine the significance of the errors associated with these models. Three mechanistic models were used to determine the adsorption mechanism. Thermodynamic parameters such as Gibbs free energy change (ΔG), enthalpy change (ΔH), entropy change (ΔS) and the activation energy (Ea) were evaluated. The adsorption process was optimized using the response surface methodology (RSM) and the artificial neural network (ANN). Packed bed adsorption column was performed to determine the effects of influent concentration, flowrate, particle size and bed height. Seven kinetic models were employed in describing the kinetics of the continuous adsorption process. BET surface area of CCAC was 903.7 m2/s while that of RHAC was 417.7 m2/s. Effects of operational parameters showed that the removal efficiency of phenol increased with increase in adsorbent dosage and contact time but decreased with temperature and initial phenol concentration. Adsorption kinetic process was best described by pseudo-second-order. The Langmuir and Flower-Guggenhein best fitted the adsorption equilibrium data. Mechanistic modeling showed that external mass transport mechanism was the rate controlling step. The ΔG ranged from -7.7 to -11.2 K/mol while ΔH ranged from 13.96 to 14.83KJ/molfor the adsorbents, suggesting that the adsorption process is spontaneous and endothermic respectively. The optimum conditions for the uptake of phenol for RHAC were dosage of 0.8g, contact time of 70.8 minutes, phenol concentration of 150 mg/l and temperature of 50 oC while for CCAC, the optimum conditions were dosage of 1.5g, temperature of 50 oC, contact time of 90 minutes and phenol concentration of 100 mg/l. This gave maximum adsorption efficiency of 92.6% and 93.5% for RHAC and CCAC respectively. Quadratic model best fitted the optimization process. ANN gave a good correlation of 0.9959 for validation of the optimum result with predicted maximum adsorption of 92.3% using CCAC and 93.4 for RHAC. Flowrate, influent phenol concentration and bed height affect the removal of phenol in the column adsorption. Wolborska and Clark kinetic models best described the column adsorption process. The study has shown that CCAC and RHAC can effectively be used as adsorbents in the uptake of phenol from aqueous solution.
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